1. Field of Invention
The present invention relates to a package for a semiconductor light emitting device and, in particular, to a package including a substrate with a thick copper layer.
2. Description of Related Art
Semiconductor light emitting devices such as light emitting diodes (LEDs) are among the most efficient light sources currently available. Material systems currently of interest in the manufacture of high brightness LEDs capable of operation across the visible spectrum include group III-V semiconductors, particularly binary, ternary, and quaternary alloys of gallium, aluminum, indium, and nitrogen, also referred to as III-nitride materials; and binary, ternary, and quaternary alloys of gallium, aluminum, indium, and phosphorus, also referred to as III-phosphide materials. Often III-nitride devices are epitaxially grown on sapphire, silicon carbide, or III-nitride substrates and III-phosphide devices are epitaxially grown on gallium arsenide by metal organic chemical vapor deposition (MOCVD) molecular beam epitaxy (MBE) or other epitaxial techniques. Often, an n-type region is deposited on the substrate, then an active region is deposited on the n-type region, then a p-type region is deposited on the active region. The order of the layers may be reversed such that the p-type region is adjacent to the substrate.
FIG. 1 illustrates a packaged semiconductor LED, described in more detail in U.S. Pat. No. 6,333,522. A III-nitride LED element 1 is formed on a transparent substrate 1a mounted face-down on a Si diode element 2 formed in a silicon substrate. Electrical connections between LED element 1 and diode element 2 are provided by gold microbumps 11 and 12 between a p-side electrode 5 of LED element 1 and an n-side electrode 8 of the Si diode element 2, and between an n-side electrode 6 of LED element 1 and a p-side electrode 7 of the Si diode element 2. The Si diode element 2 functions to protect LED element 1 from electrostatic discharge. The Si diode element 2 has a backside electrode 9 connected to a mount portion 15 of a lead frame 13a by a conductive paste 14. The p-side electrode 7 of the Si diode element 2 has a bonding pad portion 10 connected to a lead frame 13b by a gold wire bond 17. A transparent resin 18 covers LED element 1 and Si diode element 2.
The package illustrated in FIG. 1 has several disadvantages. The package of FIG. 1 does not efficiently conduct heat away from LED element 1. Heat is generated within the LED during regular operation. Light is generated in the LED by electrons from the n-type region recombining with holes from the p-type region. Some of this recombination is radiative, leading to the emission of photons. A sizeable fraction of the recombination may be non-radiative, generating heat instead of photons. In addition, some of the photons generated by radiative recombination are absorbed within the device, creating additional heat. In some devices, at least some of the heat generated within the device must be conducted away from the die to avoid damaging the LED. Neither resin 18 nor wire bond 17 conducts a significant amount of heat away from the LED. Thus, the only path conducting heat away from the device is lead 13a. The limited cross section and long length of lead 13a limits the amount of heat that can be eliminated from the LED through this path. The inability of the package of FIG. 1 to conduct heat away from LED element 1 can lead to hot spots and isolated device failures.
In addition, the package of FIG. 1 does not easily allow for multiple devices, such as multiple LED elements or chips containing other circuitry, to be connected to lead frames 13a and 13b. 
Historically, LEDs have operated at low power, for example, less than 300 mW. The problems with the package of FIG. 1 become severe design limitations as newer generations of LEDs are planned to be operated at higher power, for example, 1 W to 500 W, and in higher temperature environments, leading to an increase in operating temperatures and heat production.